Metals, such as foundry iron, used for casting and steel making, are produced in the metal industry by a number of different processes. One process of producing foundry iron utilizes a standard cupola-type furnace. A variety of iron sources are fed into the vertical shaft of the furnace fueled by combustion of coke by a blast of air. The charge added to the furnace generally contains a number of additives such as ferrosilicon to increase the silicon content of the iron and slag forming materials such as limestone to remove impurities such as sulfur.
The cupola-type furnace is a net silicon oxidizer whereby as much as 30 percent of the available silicon charged is lost by oxidation and discharged in the slag. Typically, only about 70 percent of the available silicon charged reports to the iron. Silicon is an essential element of foundry iron and is typically added in the form of ferrosilicon since such a form of silicon is readily combinable with the iron.
The feasibility of producing metals including foundry iron is dependent in part on the efficiency of the process used and cost of the charging materials. The cost of scrap iron and scrap steel depends on several factors including the iron content, amounts of desirable and undesirable alloy constituents present, and the particle size. The use of light scrap such as borings or turnings in a cupola requires agglomeration or briquetting since the high volume of gases exiting the cupola otherwise carries an unacceptably large percentage of the charge from the furnace.
Foundry iron is also produced with the electric induction furnace. A charge is heated and then additives, including silicon, carbon, and a slag forming material are introduced to cover the iron. The iron charge is heated by eddy currents resulting from electromagnetic induction from the alternating electric current flowing in the coil surrounding the charge. Silicon is typically added as ferrosilicon, and carbon is added in the form of a low sulfur content graphite material. The resulting iron generally has a silicon content of about 1-3 percent and a carbon content of about 2-4 percent.
Foundry iron has been produced commercially in standard-type electric arc furnaces (EAF). The EAF typically consists of a refractory lined vessel or shell with a removable refractory roof through which three electrodes in a three phase AC furnace, or one electrode in a DC furnace protrude into the space above the charge material and bath contained within the furnace shell. For DC furnaces, the return electrode is typically built into the bottom of the furnace shell.
The operation of the electric arc furnace typically involves charging the furnace by swinging the roof aside and emptying one or more charge buckets containing iron or steel scrap and other materials into the shell, closing the roof, and then lowering the electrodes until contact is made with the charge and arcing and melting of the charge occurs. After melting, a slag layer is usually established for refining purposes, and additions of ferrosilicon and carbon are made until the foundry iron composition reaches the desired target.
In recent years, the EAF has not been used extensively for production of foundry iron alloys because of the relatively high production cost. The EAF has been mostly limited by economics to the production of special alloy foundry irons and steels, which may not be readily or economically produced in either cupolas or induction furnaces.
Foundry iron is also produced by smelting iron ore in a submerged arc electric furnace. Submerged arc furnaces have an advantage of directly smelting the ores along with simultaneous carbothermic chemical reduction of metal oxides by the carbonaceous reducing agents, such as coke and coal. The electrodes are immersed in the charge and slag layer which forms above the molten iron. This arrangement permits efficient heat transfer between the arc and charge materials. However, the electrical conductivity of the charge must be controlled to permit the simultaneous immersion of the electrodes deep into the charge while avoiding excessive currents and overheating of the electrodes.
One example of the use of a submerged arc furnace to smelt iron ore is disclosed in U.S. Pat. No. 4,613,363 to Weinert. The carbothermic reduction of ores to produce iron requires large amounts of electric energy, thereby increasing the production costs. Alternatively, the more widely utilized processes of producing foundry iron (cupola and induction furnaces) require comparatively expensive starting materials and silicon carbide or ferrosilicon. All of these characteristics have limited these prior processes for producing foundry iron.
The electric arc furnace, such as the submerged arc furnace or open arc furnace, can be a cost-effective method of producing molten metals. For example, U.S. Pat. No. 5,588,982 to Hendrix discloses a process for efficiently producing foundry iron in an electric arc furnace by melting scrap metal while smelting an oxide such as silica. However, producing molten metals from a highly conductive charge, such as when the charge contains scrap metal, the present electric arc furnaces are inherently inefficient as a result of the construction of the electrode and particularly the cathode. Typically, the electrode is an uninsulated conductive rod of metal or metal alloy, graphite, or carbon. The electrode is provided with tapped ends for connecting several electrodes together and feeding the electrodes into the electric arc furnace during the melting process. The arc is produced at the tip of the electrode where the most efficient heating occurs. However, when the charge in the electric arc furnace is highly conductive, an open arc condition is created. This leads to inefficient heating of the charge and inefficient use of electricity.
Arc furnaces have been used to melt scrap metal as disclosed in U.S. Pat. No. 5,555,259 to Feuerstache. The furnace is formed with a center pipe surrounding the cathode which prevents the charge from contacting the side of the cathode. An arc is formed between the exposed end of the cathode and the metal bath, which is in contact with an anode for melting the charge. The lower end of the pipe is tapered for feeding the scrap to the cathode. The pipe surrounding the cathode enables the cathode to be positioned deep in the charge bed. This construction has the disadvantage of including a water-cooled, fixed, non-consumable barrier between the electrode and the charge.
Accordingly, the metals industry has a continuing need for an economical and efficient process for producing various metal alloys in an electric furnace.